1. Field of the Invention
The invention relates to gradient index glasses obtained through a sol-gel process.
2. Discussion of the Background
Gradient index (GRIN) optical materials have become a prominent part of modern optics. GRIN materials are characterized by a refractive index distribution which varies spatially in a controlled manner.
For example, cylindrical glass rods with parallel plane faces can be made with a refractive index higher along the central axis than at the outer edges, i.e., the index changes in a radial fashion with the highest index centrally located along the axis and the lowest index located at the outer surface. The change in index from the axis to the outer surface is referred to as the delta-n (.DELTA.n). These rods can be used to focus light despite their plane end faces because light follows a curved path within the material. Illustratively, TiO.sub.2 :R'.sub.2 O:SiO.sub.2 GRIN glasses have been reported in which the gradient is caused by a gradient of two or more R'.sub.2 O components, such as Ag.sub.2 O:Na.sub.2 O or Ti.sub.2 O:K.sub.2 O or Cs.sub.2 O:K.sub.2 O.
GRIN lenses have received attention in recent years because of their application in fiber optics, photocopiers, fax equipment, and so on. A graph of index of refraction versus spatial position is commonly referred to as the index profile. The manner in which the index profile changes with different wavelengths of light is called the index profile dispersion. Delta-n, the shape of the index profile and the index profile dispersion are the most important characteristics of a gradient index glass.
The use of gradient index glass in optical systems provides many advantages over homogeneous glass. These advantages include improved performance and greater simplicity by reducing the total number of optical elements needed in a system.
In general, GRIN designs require fewer elements than their homogeneous counterparts. Thus, definite benefits exist with respect to size, weight, and economics when using GRIN materials in fiber and integrated (miniaturized systems) optical applications. Furthermore, some systems can be designed using GRIN lenses which would be virtually impossible to configure using homogeneous lenses.
Several methods are known for making GRIN optical materials. These include ion exchange in solid glass, stuffing/unstuffing of porous glass, phase separation/leaching and sol-gel techniques. All these methods are similar in that they involve immersion of a substrate in a liquid phase to induce diffusion of index modifying components in order to create a composition gradient within the substrate.
The ion exchange method, which has been commercially developed for the production of GRIN lenses involves exchanging ions from a molten salt bath with those in a dense glass. This method, along with others, has been described by Mukherjee in: "Gradient Index Lens Fabrication Processes: A Review, " Proceedings of a Topical Meeting on Gradient-Index Optical Imaging Systems, May 4-5, 1981, Honolulu, Hi, Optical Society of America, pp. TuAl-1 to TuAl-5, 1981. Drawbacks of materials produced by ion exchange techniques include small size, poor environmental and thermal stability, toxicity owing to ions like Tl, and a limited choice of index modifying ions which limits the variety of optical characteristics.
In the stuffing/unstuffing techniques, a porous glass preform, made by leaching a phase separated glass in acid, is stuffed with index modifying ions such as Cs.sup.+ or Tl.sup.+ by infiltration and precipitation of the analogous salt solutions. A concentration gradient of the modifier ions is then created by redissolving the salt and allowing it to diffuse out of the preform. The diffusion process is halted by precipitation after the desired composition profile is achieved, and the preform is dried and sintered. Such a method is described in U.S. Pat. Nos. 3,938,974, 4,302,331 and 4,640,699.
A drawback of this method is that it is difficult to produce small diameter GRIN rods in a consistent manner, due to the short diffusion times of the modifying ions. Furthermore, though this technique can be used to prepare lenses of 10 mm aperture, these lenses often exhibit index gradients which are not uniform because of the non-uniform pore size distribution created in the porous glass during the phase separation and leaching processes.
The "phase separation and leaching" technique is similar to the stuffing/unstuffing method in that the initial steps involve phase separating a suitable glass by heat treatment and then leaching away the soluble phase in an acid solution (Physics and Chemistry of Glasses 21, 22-24, 1980). In this case, however, the starting glass contains a significant amount of germanium dioxide which is not completely removed during leaching. A spatial variation in the concentration of GeO.sub.2 is thus created by the processes of dissolution and diffusion. After leaching, the gel is washed, dried and sintered.
The primary disadvantage of the "phase separation and leaching" technique is that the selection of phase separable glasses is very limited.
Recently, researchers have been pursuing a number of avenues for making gradient index glass which utilize porous silicate preforms fabricated by sol-gel methods.
The potential advantages of using sol-gel precursors in the production of gradient index glass include: (1) relatively large diffusion coefficients; (2) low energy consumption during most of the process; (3) the ability to introduce a broad variety of index modifying dopants into the sol-gel preform; and (4) multi-component compositions can be formed into glasses of different sizes and shapes. The fabrication of a GRIN glass rod by sol-gel processes is especially advantageous for the manufacture of GRIN materials of large size and large variation of index.
Yamane (U.S. Pat. No. 4,686,195) produced a gradient index glass by a sol-gel technique. This technique involves mixing a silicon alkoxide with water, a source of boron oxide, and an aqueous metal salt solution which is the source of modifier cations. This mixture forms a gel which then is placed in a solution to leach out some of the metal salts contained within it and to have other metal salts introduced into it by diffusion. The gel then is dried and sintered into glass. The main problem with this technique is that since the index modifiers are introduced as salts they are not incorporated into the structural network of the gel until it is heated to higher temperatures. The modifier cations are thus free to migrate during the drying step, and this causes asymmetry in the final index profile.
Shingyouchi et al (Electronic Letters, 22:99-100 and 1108-1110, 1986), reported a different technique in which germanium as the index modifying cation, is introduced as an alkoxide rather than as a salt. The index modifier thus is fully incorporated into the gel structure, and the index profile does not suffer from uncontrollable asymmetry.
In this technique, tetramethoxysilane (a silicon alkoxide) is first combined with tetraethoxygermanium (a germanium alkoxide), ethanol, water and hydrochloric acid. The mixture forms a gel which is placed in water to leach out some of the germanium component. The gel is washed in methanol to fix the germanium concentration gradient, and then dried and sintered into gradient index glass.
Shingyouchi et al also used titanium to replace germanium as the index modifying cation. The resulting glass was a rod 2 mm in diameter with a .DELTA.n of 0.013.
The method of Shingyouchi et al involves the use of only two components: silica and an index modifying oxide, such as germanium dioxide or titanium dioxide. The method can be generalized to substitute zirconium dioxide as well.
These binary systems, however, yield gels which shrink considerably during drying. This large shrinkage results in a dense gel which is difficult to sinter without fracturing or bloating. Binary SiO.sub.2 -TiO.sub.2 also tends to crystallize at elevated temperatures if the TiO.sub.2 content exceeds 4 to 5 mole percent. Binary SiO.sub.2 -ZrO.sub.2 gels tend to bloat at elevated temperatures because the outside portions of the gel collapse before the inside portions, thereby trapping any internal gasses generated during heating. As a result, the method and compositions disclosed by Shingyouchi et al suffer from several flaws.
The flaws of the above existing techniques and compositions have been avoided in U.S. Pat. No. 4,797,376. Asymmetry of the index profile has been avoided by introducing the metal oxide precursor as an alkoxide instead of as a salt; and the difficulties inherent in binary metal oxide sol-gel systems have been avoided by using three or more components.
An alkoxide of silicon and at least two different metal alkoxides are added to form a mixture. The first of the alkoxides acts as an index modifier and is selected from the group consisting of alkoxides of titanium and zirconium. The second of the alkoxides acts as a gel modifier and is selected from the group consisting of alkoxides of boron, aluminum and germanium. The resulting ternary or greater solution is allowed to gel. The gel is placed in a leaching bath, dried, and then sintered to form a glass.
This process is however limited in the amount of index modifier (TiO.sub.2 or ZrO.sub.2) which can be incorporated in the final glass due to the problem of devitrification.
Moreover, one major problem with gradient index glass made by leaching alkoxide gels is that the maximum index change is small. This severely restricts the range of commercial applications.